Many wall mirrors are large and quite heavy and it is desirable to support such mirrors on the wall studs. However, the studs are frequently not located in the exact position where it is desired to locate the mirror mounting brackets, and it has heretofore been proposed for example as shown in U.S. Pat. Nos. 2,696,962 and 3,237,898, to provide elongated mounting strips having a length to span at least a pair of adjacent studs, and to mount the mirror mounting brackets on the mounting strip. It is necessary that at least some of the mirror mounting brackets be adjustable in a direction crosswise of the mounting strip, to enable movement of the mirror engaging hooks on the mounting brackets into engagement with the edge of the mirror, after the mirror has been positioned. In U.S. Pat. No. 2,696,962, a guide member was attached by rivets at a selected location on the mounting strip to provide guideways extending crosswise of the strip for adjustably receiving the mirror mounting bracket. This construction utilized a number of separate parts and required an assembly operation to mount the bracket guide member on the strip, and this significantly increased the overall cost of the mirror mounting device. In U.S. Pat. No. 3,237,898, the face of the mounting strip was transversely slotted and stamped or formed to provide transversely disposed guide flanges spaced from the front of the mounting strip for slidably receiving the mirror mounting bracket. The transverse slotting of the mounting strip, however, markedly weakened the mounting strip. U.S. Pat. Nos. 2,696,962 and 3,237,898 do disclose the provision of longitudinal ribs or flutes in the mounting strip in order to stiffen and rigidify the same. However, in both of these patents, the mirror mounting bracket is disposed at the front side of the mounting strip and the depth of the stiffening ribs or flutes in the mounting strip add to the spacing between the wall and the mirror mounting bracket and undesirably increase the spacing between the rear side of the mirror and the wall.